Last night I had to add an additional vCPU to a Windows 2008 R2 guest. I tried and tried to research doing this, but all I was able to find was how to Hot add a vCPU, which I didn't care to do. Also all of the VMWare documentation said to view a certain PDF to see what OSes could even take a Hot CPU addition, but I was unable to find anywhere in the PDF what OSes could handle it and which ones couldn't.
So I installed the newest patches (being patch Tuesday and all) and I shut down the server. I then went from one vCPU to two vCPUs. I ignored the warning that adding CPUs to an already installed system may cause it to be unstable.
So far the server has been stable and everything worked like I had hoped. I don't know if this would work on any of our Windows 2008 servers, or for that matter the CentOS 5.5 servers, but all of them already have two vCPUs.
If you are wondering why I needed to do this, well, it is because our VoIP recording solution uses MySQL, and for some reason it has been using 100% of the CPU for weeks now. This despite a mostly empty process list in MySQL.